audio: Implement getters for hardware mixer controls

The VTS test for volume accessors first determines whether
the accessor is supported by calling the getter. The getter
must return `UNSUPPORTED_OPERATION` if the control is not
supported. For this reason, wire the getter to the hardware
mixer implementation.

Bug: 302132812
Test: atest VtsHalAudioCoreTargetTest
Change-Id: Ia50def0d076b6d3c46db55123186eab34bbbb954